The present invention relates to a piezoelectric acceleration sensor to be used for detecting impact and acceleration applied to a body, more particularly an acceleration sensor for detecting characteristic quantity generated by an inertia force resulting from the acceleration.
In recent years, most electronic equipment is becoming smaller in size. Portable equipment such as a notebook personal computer (PC) is widely used today. There may be a case that such equipment receives an unpredictable impact. It is therefore required to detect the impact so that appropriate measures can be taken for maintaining reliable operation of the equipment.
For example, on a hard disk drive (HDD) mounted in a notebook PC or a desk top PC, an acceleration sensor is being used to prevent a drive read/write error possibly produced when receiving impact. In particular, in case of a notebook PC, an HDD usually receives acceleration in a vertical direction against an HDD face caused by keyboard typing, as well as in a parallel direction to the HDD face caused by read/write operation of the head. Therefore it is necessary to detect acceleration in either direction mentioned above.
As equipment becomes smaller in size with higher performance, a smaller sized, higher performance sensor capable is correspondingly required for detecting acceleration in two or more axial directions; an axial direction along a plane and an axial direction perpendicular to that plane.
Conventionally, a piezoelectric acceleration sensor capable of detecting biaxial acceleration are known. A method for detecting biaxial acceleration is disclosed in the official gazette of Japanese Unexamined Patent Publication No. Hei-7-20144, in which an acceleration sensing element is mounted obliquely against the bottom face of an equipment case. Other method for detecting biaxial acceleration is disclosed in the official gazette of Japanese Unexamined Patent Publication No. Hei-11-118823, in which a vibrator is obliquely attached on a support body at a predetermined angle against a main face.
Furthermore, in the official gazette of Japanese Unexamined Patent Publication No. Hei-8-43432, there is disclosed a method for detecting biaxial acceleration by providing a polarization of a piezoelectric ceramics disposed obliquely to the vertical direction from a ceramics plane. Also, in the official gazette of Japanese Unexamined Patent Publication No. Hei-11-211748, there is disclosed another method for detecting biaxial acceleration by constituting a weight at the end of a vibrator in an eccentric position against the axial direction.
However, these conventional methods have problems described below: According to the method of obliquely mounting an acceleration detection element, the implementation becomes complicated and costly. The method of providing a support body attached at an angle becomes large in size in a vertical direction and also requires complicated mounting. In the method of providing obliquely disposed polarization, it is necessary to cut a vibrator in a desired direction after being polarized, then to form an electrode thereafter. This causes increased manufacturing process and cost.
Also, the method of forming a weight at the end of a vibrator has a problem that the divergence in forming position causes increased dispersion in sensitivity.
To solve the aforementioned problems, a configuration of an acceleration sensor has been proposed by the applicant of the present invention (refer to the official gazette of Japanese Unexamined Patent Publication No. Hei-2000-97707 and No. Hei-11-375813.)
According to this proposed method, an acceleration sensor provides a vibrator and a weight connected thereto having such a configuration that the weight is supported at a position deviating from the center of gravity of the total body of the vibrator and the weight. With this configuration, rotation moment is generated at the weight when acceleration is applied thereto. By detecting the characteristic quantity in the vibrator (torsion) corresponding to the generated rotation moment, the amount of acceleration can be obtained.
The inventors of the present invention have been further developing a small and high sensitive acceleration sensor without requiring large-sized vibrator.
Accordingly it is an object of the present invention to provide an acceleration sensor having higher performance in terms of either directional or non-directional detection sensitivity, with a similar configuration to the acceleration sensor previously proposed by the applicants of the present invention.
The basic configuration of the present invention provides a vibrator having uni-directional polarization and a weight connected to the vibrator. The weight is supported at a position deviating from the center of gravity of the entire body consisting of the vibrator and the weight. Further, the polarization axis of the vibrator is formed in a different direction from the torsion axis of the vibrator.
Also, as another aspect of the basic configuration of the present invention, there are provided a vibrator having unidirectional polarization and a weight connected to the vibrator, while the weight is supported at a position deviating from the center of gravity of the entire body consisting of the vibrator and the weight. Here, the polarization axis is formed in a direction identical to the axis of torsion of the vibrator.
In each aforementioned configuration, preferably the vibrator is provided with a plurality of separately disposed detection electrodes to attain the aforementioned object.
Further, preferably the area of one electrode among the plurality of separately disposed electrodes is different from the other electrodes.
Still further, preferably the vibrator and the weight are positioned such that each center position of line symmetry in a width direction mutually deviates.
Still further, preferably the center of gravity of the weight is positioned deviating from the center position of line symmetry in a width direction of the vibrator.
Still further, preferably the vibrator is a piezoelectric vibrator formed of either a piezoelectric single crystal or a piezoelectric polycrystal.
Still further, preferably the separated electrodes of the vibrator is formed through etching, dicing or the like.
Still further, preferably the vibrator is provided with two detection electrodes formed by dividing in parallel with a direction of torsion of the vibrator.
Still further, preferably the vibrator is provided with two detection electrodes which are separated perpendicularly to a torsion direction of the vibrator.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments accompanied by drawings.